1. Field of the Invention
The present invention relates in general to an electric heater used for heating a workpiece of a small size, which has both a portion which must be heated and a portion which must not be heated, and a method of heating the workpiece by means of such an electric heater. More particularly, the present invention is concerned with an electric heater which is suitably used for firing a protective glass for a platinum-film resistor used in a film element of an air-flow sensor, for example.
2. Discussion of the Prior Art
A platinum-film resistor is generally used in a film element of an air-flow sensor for detecting a flow rate of air stream. The platinum-film resistor is produced by attaching a pair of lead wires made of base metal, to opposite ends of a ceramic body having a platinum layer formed thereon by plating.
In producing the platinum-film resistor, the platinum plating layer formed on the ceramic body is secured by firing thereof to the ceramic body, and the base metal lead wires are connected by firing thereof to the platinum plating layer. Thereafter, the platinum plating layer on the ceramic body is coated with a glass, and the applied glass is fired, whereby a protective glass for protecting the platinum plating layer is formed on the ceramic body. It is generally recognized that the entire length of the platinum plating layer (ceramic body) to which a glass coating is applied is extremely short, e.g., in the neighborhood of 4-5 mm. Further, no means is known for effectively heating only the platinum plating layer coated with the glass. Under these situations, the firing of the glass has been conventionally effected by using a tubular furnace, which is constructed such that a heat generating element is wound around a ceramic tube made of alumina, for example, and which is adapted to heat the whole platinum-film resistor accommodated therein. Alternatively, the glass on platinum plating layer is fired in a box-like furnace which has a box-like body made of brick, on which a heat generating element formed of SiC, for example, is provided.
However, when the firing of glass is effected by heating the whole platinum-film resistor in the tubular or box-like furnace, it takes a long time for the furnace to be heated or cooled to a desired degree, since the furnace has a relatively large heat capacity. Further, since the firing temperature of the glass is extremely high, e.g., in the neighborhood of 800.degree. C., the temperature in the furnace is maintained for a considerably long time above the temperature at which the lead wires are oxidized, (usually 400.degree. C. or higher). Therefore, if only the platinum plating layer on the ceramic body is coated with a glass, and the glass is fired in the furnace with the lead wires being exposed to the atmosphere in the furnace, the lead wires undergo a change of color due to oxidation, and are even torn off in the worst case. For preventing the oxidation of the lead wires, nitrogen or hydrogen gas may be introduced into the furnace, so that the furnace contains therein a neutral or reducing atmosphere. In this case, however, the atmosphere in the furnace is likely to decompose the glass on the platinum plating layer, or give rise to bubbles in the glass. Thus, it is found difficult to employ the method of controlling the atmosphere in the furnace in the manner as described above.
In view of the above, the lead wires as well as the platinum plating layer on the ceramic body are generally coated with a glass, so that the glass coating formed on the lead wires prevents the lead wires from being oxidized during firing of the glass. This method requires the procedure of removing the fired glass covering the lead wires, after the resistor is taken out from the furnace. Thus, the method suffers from considerable reduction in the productivity of the platinum-film resistor.
In the conventional method using the tubular or box-like furnace which has a relatively large heat capacity, it is difficult to control the temperature in the furnace, at a constant level in a medium to low temperature range below about 800.degree. C. Namely, the furnace is likely to suffer from an overshoot or ringing phenomenon, deterioration in heat efficiency, and an increased firing time.
An electric heating device may be used instead of the furnace as described above. However, the electric heating device is incapable of uniformly heating the workpiece, thereby causing local variation in the heating temperature of the workpiece. The electric heating device also suffers from an extremely large heat capacity, as compared with that of the platinum-film resistor.